The present invention relates generally to high-pressure, air supply systems such as those used in connection with fluidized bed boilers and, more particularly, to a variable resistance device for use in such systems for controlling the air flow therethrough at desired pressures.
Air supply systems employ an arrangement of ductwork and one or more fans to provide air at a desired delivery pressure and flow rate. In many cases, the fan installed in the air supply system has a larger flow rate capacity and/or delivery pressure than initially required so that it can accommodate a future operating condition. One such example would be a situation where a fan used to supply air for a boiler is sized for a higher boiler load condition, but the fan will be operated at a lower boiler load condition in the interim. In such situations, the fan must be stabilized so that it can operate at the reduced system load operating point corresponding to the reduced boiler load condition. This is usually accomplished by adding air resistance or pressure drop into the system, in lieu of making expensive fan modifications or replacing the fan itself. The most cost effective and common way to add such air resistance or pressure drop to an air supply system is to install a stationary perforated plate or a standard flow damper in the ductwork.
A situation arose in connection with a fluidized bed boiler system where additional resistance was needed to stabilize an oversized forced draft fan used to fluidize the bed. The forced draft fans used in such service are high static pressure fans (producing near 70 inches water column) and are designed to supply air to the bottom of the fluidized bed boiler. The high static pressure is required because these fans are responsible for the fluidization of the bed material. The other distinct aspect of this application is that this fan is required to change airflow as needed to match the boiler load while still maintaining sufficient static pressure to fluidize the bed material. As it turned out, the actual operating condition required less static pressure than had been estimated, and caused the fan to operate below its design static pressure curve in an unstable manner. The fan instability caused a surging effect at the fan that resulted in a combustion pulse in the boiler. This combustion pulse was, in turn, amplified in strength more than ten times. This amplification was believed to be caused by the fluidized bed boiler geometry. On site testing confirmed that adding resistance to the air system stabilized the fan. Through this testing both a high and low airflow resistance parameter was developed. These resistance parameters were unique in that the high airflow condition required less resistance than the low flow condition.
Both stationary perforated plate and conventional flow damper schemes were explored and found inadequate for this application. The stationary perforated plate could not provide the desired resistance at both high and low airflows. The conventional flow damper was not able to offer stable operation at the low airflow condition. In fact, due to the inherent characteristics of a flow control device operated at more than 50% closed, it was very likely that a conventional flow damper could itself create a pressure fluctuation at the low airflow condition. What was needed was a device that could vary the resistance with airflow demands and provide stable operation at all airflow conditions. The present invention provides a solution to this unique problem.